Kinship is a relationship between any entities that share a genealogical origin, through either biological, cultural, or historical descent. And descent groups, lineages, etc. are treated in their own subsections.

In anthropology the kinship system includes people related both by descent and marriage, while usage in biology includes descent and mating. Human kinship relations through marriage are commonly called "affinity" in contrast to "descent" (also called "consanguinity"), although the two may overlap in marriages among those of common descent. Family relations as sociocultural genealogy lead back to gods[1] (see mythology, religion), animals that were in the area or natural phenomena (as in origin stories).

Kinship is one of the most basic principles for organizing individuals into social groups, roles, categories, and genealogy. Family relations can be represented concretely (mother, brother, grandfather) or abstractly after degrees of relationship. A relationship may have relative purchase (e.g., father is one regarding a child), or reflect an absolute (e.g., status difference between a mother and a childless woman). Degrees of relationship are not identical to heirship or legal succession. Many codes of ethics consider the bond of kinship as creating obligations between the related persons stronger than those between strangers, as in Confucian filial piety.

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One of the founders of the anthropological relationship research was Lewis Henry Morgan, in his Systems of Consanguinity and Affinity of the Human Family (1871). Members of a society may use kinship terms without all being biologically related, a fact already evident in Morgan's use of the term affinity within his concept of the "system of kinship". The most lasting of Morgan's contributions was his discovery of the difference between descriptive and classificatory kinship, which situates broad kinship classes on the basis of imputing abstract social patterns of relationships having little or no overall relation to genetic closeness but do reflect cognition about kinship, social distinctions as they affect linguistic usages in kinship terminology, and strongly relate, if only by approximation, to patterns of marriage.[2] The major patterns of kinship systems which Lewis Henry Morgan identified through kinship terminology in his 1871 work Systems of Consanguinity and Affinity of the Human Family are:

The six types (Crow, Eskimo, Hawaiian, Iroquois, Omaha, Sudanese) that are not fully classificatory (Dravidian, Australian) are those identified by Murdock (1949) prior to Lounsbury's (1964) rediscovery of the linguistic principles of classificatory kin terms.

The concept of “system of kinship” tended to dominate anthropological studies of kinship in the early 20th century. Kinship systems as defined in anthropological texts and ethnographies were seen as constituted by patterns of behavior and attitudes in relation to the differences in terminology, listed above, for referring to relationships as well as for addressing others. Many anthropologists went so far as to see, in these patterns of kinship, strong relations between kinship categories and patterns of marriage, including forms of marriage, restrictions on marriage, and cultural concepts of the boundaries of incest. A great deal of inference was necessarily involved in such constructions as to “systems” of kinship, and attempts to construct systemic patterns and reconstruct kinship evolutionary histories on these bases were largely invalidated in later work. However, anthropologist Dwight Read later argued that the way in which kinship categories are defined by individual researchers are substantially inconsistent.[3] This occurs when working within a systemic cultural model that can be elicited in fieldwork, but also allowing considerable individual variability in details, such as when they are recorded through relative products.[4]

In trying to resolve the problems of dubious inferences about kinship "systems", George P. Murdock (1949, Social Structure) compiled kinship data to test a theory about universals in human kinship in the way that terminologies were influenced by the behavioral similarities or social differences among pairs of kin, proceeding on the view that the psychological ordering of kinship systems radiates out from ego and the nuclear family to different forms of extended family. Lévi-Strauss (1949, Les Structures Elementaires), on the other hand, also looked for global patterns to kinship, but viewed the “elementary” forms of kinship as lying in the ways that families were connected by marriage in different fundamental forms resembling those of modes of exchange: symmetric and direct, reciprocal delay, or generalized exchange.

A more flexible view of kinship was formulated in British social anthropology. Among the attempts to break out of universalizing assumptions and theories about kinship, Radcliffe-Brown (1922, The Andaman Islands; 1930, The social organization of Australian tribes) was the first to assert that kinship relations are best thought of as concrete networks of relationships among individuals. He then described these relationships, however, as typified by interlocking interpersonal roles. Malinowski (1922, Argonauts of the Western Pacific) described patterns of events with concrete individuals as participants stressing the relative stability of institutions and communities, but without insisting on abstract systems or models of kinship. Gluckman (1955, The judicial process among the Barotse of Northern Rhodesia) balanced the emphasis on stability of institutions against processes of change and conflict, inferred through detailed analysis of instances of social interaction to infer rules and assumptions. John Barnes, Victor Turner, and others, affiliated with Gluckman’s Manchester school of anthropology, described patterns of actual network relations in communities and fluid situations in urban or migratory context, as with the work of J. Clyde Mitchell (1965, Social Networks in Urban Situations). Yet, all these approaches clung to a view of stable functionalism, with kinship as one of the central stable institutions.

Building on Lévi-Strauss’s (1949) notions of kinship as caught up with the fluid languages of exchange, Edmund Leach (1961, Pul Eliya) argued that kinship was a flexible idiom that had something of the grammar of a language, both in the uses of terms for kin but also in the fluidities of language, meaning, and networks. His field studies criticized the ideas of structural-functional stability of kinship groups as corporations with charters that lasted long beyond the lifetimes of individuals, which had been the orthodoxy of British Social Anthropology. This sparked debates over whether kinship could be resolved into specific organized sets of rules and components of meaning, or whether kinship meanings were more fluid, symbolic, and independent of grounding in supposedly determinate relations among individuals or groups, such as those of descent or prescriptions for marriage. Work on symbolic kinship by David M. Schneider in his (1984, A Critique of The Study of Kinship) reinforced this view. In response to Schneider's 1984 work on Symbolic Kinship, Janet Carsten re-developed the idea of "relatedness" from her initial ideas, looking at what was socialized and biological, from her studies with the Malays (1995, The substance of kinship and the heat of the hearth; feeding, personhood and relatedness among the Malays in Pulau Langkawi, American Ethnologist). She uses the idea of relatedness to move away from a pre-constructed analytic opposition which exists in anthropological thought between the biological and the social. Carsten argued that relatedness should be described in terms of indigenous statements and practices, some of which fall outside what anthropologists have conventionally understood as kinship (Cultures of Relatedness, 2000). This kind of approach – recognizing relatedness in its concrete and variable cultural forms – exemplifies the ways that anthropologists have grappled with the fundamental importance of kinship in human society without imprisoning the fluidity in behavior, beliefs, and meanings in assumptions about fixed patterns and systems.

Ideas about kinship do not necessarily assume any biological relationship between individuals,
rather just close associations. Malinowski, in his ethnographic study of sexual behaviour on the Trobriand Islands noted that the Trobrianders did not believe pregnancy to be the result of sexual intercourse between the man and the woman, and they denied that there was any physiological relationship between father and child.[7] Nevertheless, while paternity was unknown in the "full biological sense", for a woman to have a child without having a husband was considered socially undesirable. Fatherhood was therefore recognised as a social role; the woman's husband is the "man whose role and duty it is to take the child in his arms and to help her in nursing and bringing it up";[8] "Thus, though the natives are ignorant of any physiological need for a male in the constitution of the family, they regard him as indispensable socially".[9]

As social and biological concepts of parenthood are not necessarily coterminous, the terms "pater" and "genitor" have been used in anthropology to distinguish between the man who is socially recognised as father (pater) and the man who is believed to be the physiological parent (genitor); similarly the terms "mater" and "genitrix" have been used to distinguish between the woman socially recognised as mother (mater) and the woman believed to be the physiological parent (genitrix).[10] Such a distinction is useful when the individual who is considered the legal parent of the child is not the individual who is believed to be the child's biological parent. For example, in his ethnography of the Nuer, Evans-Pritchard notes that if a widow, following the death of her husband, chooses to live with a lover outside of her deceased husband's kin group, that lover is only considered genitor of any subsequent children the widow has, and her deceased husband continues to be considered the pater. As a result, the lover has no legal control over the children, who may be taken away from him by the kin of the pater when they choose.[11] The terms "pater" and "genitor" have also been used to help describe the relationship between children and their parents in the context of divorce in Britain. Following the divorce and remarriage of their parents, children find themselves using the term "mother" or "father" in relation to more than one individual, and the pater or mater who is legally responsible for the child's care, and whose family name the child uses, may not be the genitor or genitrix of the child, with whom a separate parent-child relationship may be maintained through arrangements such as visitation rights or joint custody.[12]

It is important to note that the terms "genitor" or "genetrix" do not necessarily imply actual biological relationships based on consanguinity, but rather refer to the socially held belief that the individual is physically related to the child, derived from culturally held ideas about how biology works. So, for example, the Ifugao may believe that an illegitimate child might have more than one physical father, and so nominate more than one genitor.[13] J.A. Barnes therefore argued that it was necessary to make a further distinction between genitor and genitrix (the supposed biological mother and father of the child), and the actual genetic father and mother of the child.

Descent, like family systems, is one of the major concepts of anthropology. Cultures worldwide possess a wide range of systems of tracing kinship and descent. Anthropologists break these down into simple concepts about what is thought to be common among many different cultures.

A descent group is a social group whose members claim
common ancestry. A unilineal society is one in which the descent of an individual is reckoned either from the mother's or the father's line of descent. With matrilineal descent individuals belong to their mother's descent group. Matrilineal descent includes the mother's brother, who in some societies may pass along inheritance to the sister's children or succession to a sister's son. With patrilineal descent, individuals belong to their father's descent group. Societies with the Iroquois kinship system, are typically uniliineal, while the Iroquois proper are specifically matrilineal.

In a society which reckons descent bilaterally (bilineal), descent is reckoned through both father and mother, without unilineal descent groups. Societies with the Eskimo kinship system, like the Eskimo proper, are typically bilateral. The egocentrid kindred group is also typical of bilateral societies.

Some societies reckon descent patrilineally for some purposes, and matrilineally for others. This arrangement is sometimes called double descent. For instance, certain property and titles may be inherited through the male line, and others through the female line.

A lineage is a unilineal descent group that can demonstrate their common descent from a known apical ancestor. Unilineal lineages can be matrilineal or patrilineal, depending on whether they are traced through mothers or fathers, respectively. Whether matrilineal or patrilineal descent is considered most significant differs from culture to culture.

A clan is a descent group that claims common descent from an apical ancestor (but often cannot demonstrate it, or "stipulated descent"). If a clan's apical ancestor is nonhuman, it is called a totem. Examples of clans are found in the Chechen, Chinese, Irish, Japanese, Polish, Scottish, Tlingit, and Somali societies. In the case of the Polish clan, any notion of common ancestry was lost long ago.

A phratry is a descent group containing at least two clans which have a supposed common ancestor.

If a society is divided into exactly two descent groups, each is called a moiety, after the French word for half. If the two halves are each obliged to marry out, and into the other, these are called matrimonial moieties.
Houseman and White (1998b, bibliography) have discovered numerous societies where kinship network analysis shows that two halves marry one another, similar to matrimonial moieties, except that the two halves—which they call matrimonial sides[14] – are neither named nor descent groups, although the egocentric kinship terms may be consistent with the pattern of sidedness, whereas the sidedness is culturally evident but imperfect.[2]

The word deme is used to describe an endogamous local population that does not have unilineal descent.[15] Thus, a deme is a local endogamous community without internal segmentation into clans.

The Western model of a nuclear family consists of a couple and its children. The nuclear family is ego-centered and impermanent, while descent groups are permanent (lasting beyond the lifespans of individual constituents) and reckoned according to a single ancestor.[citation needed]

Kinship calculation is any systemic method for reckoning kin relations. Kinship terminologies are native taxonomies, not developed by anthropologists.

Beanpole family is a term used to describe expansions of the number of living generations within a family unit, but each generation has relatively few members in it.

Kinship and descent have a number of legal ramifications, which vary widely between legal and social structures.

Next of Kin traditionally and in common usage refers to the person closest related to you by blood, such as a parent or your children.

In legal terms, for example in intestacy, it has come to mean the person closest to you, which is generally the spouse if married, followed by the natural children of the deceased.

Whilst someone is alive they may nominate any person close to them to be their next of kin. The next of kin is usually asked for as a contact in case of accident, emergency or sudden death. It does not involve completing any forms or registration in the UK, and may be a friend or carer unrelated to you by blood or marriage.

More importantly, kinship and descent enters the legal system by virtue of intestacy, the laws that at common law determine who inherits the estates of the dead in the absence of a will. In civil law countries, the doctrine of legitime plays a similar role, and makes the lineal descendants of the dead person forced heirs. Rules of kinship and descent have important public aspects, especially under monarchies, where they determine the order of succession, the heir apparent and the heir presumptive.

Evolutionary psychologists currently do not argue that humans automatically know the genetic relationships between people or that culture does not affect how kinship is perceived. One current view is that humans have an inborn but culturally affected system for detecting certain forms of genetic relatedness. One important factor for sibling detection, especially relevant for older siblings, is that if an infant and one's mother are seen to care for the infant, then the infant and oneself are assumed to be related. Another factor, especially important for younger siblings who cannot use the first method, is that persons who grew up together see one another as related. Yet another may be detection based on the major histocompatibility complex (See Major Histocompatibility Complex and Sexual Selection). This kinship detection system in turn affects other genetic predispositions such as the incest taboo and a tendency for altruism towards relatives.[16]

Holland in a 2004 thesis argued that some approaches in sociobiology and evolutionary psychology have repeated earlier anthropological mistakes regarding putting too much emphasis on genetic relatedness regarding kinship, and that inclusive fitness theory does not require that genetic relatedness per se is the condition that mediates social bonding and social cooperation. In ancestral environments, social cooperation mediated through ties of familiarity and attachments would have increased the inclusive fitness of genes without any form of detection of actual genetic relationships. Inclusive fitness in this view is seen as an ultimate cause rather than a proximate cause (See Tinbergen's four questions) of social bonding and cooperation. Holland thus argues that properly understood, the findings of cultural anthropologists and the theories of evolutionary biologists are compatible.[17]

Holland also argued that the work of Janet Carsten (outlined above) represents one productive strand of the re-casting of the relationship between kinship and biology. A similar approach, that has been called 'Nurture kinship'[17][18] emphasizes that social relationships, and the cooperation that accompanies them, are commonly built upon emotional bonds and attachments. This perspective re-unites current ethnographic findings both with the work of earlier anthropologists such as Audrey Richards, and also with John Bowlby and colleagues' foundational work on emotional attachment theory. In this sense inclusive fitness theory is indeed compatible with ethnographic data on human kinship, though its explanatory scope is much narrower than sociobiology and evolutionary psychology had typically assumed. Its application is limited to theorizing the ultimate causes of the (non-deterministic) proximate mechanisms of cooperation such as emotional attachments. Bowlby himself emphasized the compatibility of his own work with inclusive fitness theory.[19] For a full account of actual kinship patterns in any particular human society, ethnography, including the analysis of e.g. symbolic, economic and other systems, remains central.